1. Field of the Invention
The invention relates to a process for subjecting materials to accelerated irradiance exposure factors that permit about a year""s worth of representative weathering to be accumulated in a period from about 3 to 10 days, under controlled weathering conditions that include several concurrent levels of temperature and/or relative humility at very high levels of natural sunlight.
In the invention process, a solar concentrator [which may include a High Flux Solar Furnace (HFSF) and an Irradiance Redistribution Guide (IRG)] is used to obtain elevated levels (25-100xc3x97) of concentrated sunlight for accelerated testing of material samples. When an IRG is used, it provides the capability of being able to modify (redistribute) the Gaussian-shaped beam from the HFSF into a more uniform profile on a sample exposure plane.
Also encompassed in the invention process for obtaining ultra-accelerated natural sunlight exposure testing is the use of reflective apparatus such as multi-step and multi-faceted concentrators and refractive apparatus such as Fresnel lens concentrators, holographic concentrators, 2D or 3D micro lens arrays, and an array of Fresnel lens facets to obtain elevated levels (25-100xc3x97) of concentrated sunlight for accelerated natural sunlight testing of material samples.
By adequately controlling sample temperatures and demonstrating that reciprocity relationships are obeyed (i.e., the level of applied accelerated stresses does not change the failure/degradation mechanism), this novel capability allows materials to be subjected to accelerated irradiance exposure factors of 25-100xc3x97, thereby permitting a year""s worth of representative weathering (in terms of natural sunlight exposure) to be accumulated in from about 3 to about 10 days.
2. Description of the Prior Art
U.S. Pat. No. 4,817,447 discloses a weathering chamber using lamps and sample temperature control using cooling air. Uniform sample irradiance at accelerated levels of up to 10 suns (within the UV bandwidth) appears attainable.
A test apparatus incorporating a mirror, which rejects infrared, is disclosed in U.S. Pat. No. 4,012,954. In the ""954 patent, convective cooling air and a conductively cooled substrate are also incorporated. However, although convective cooling is used, the air movement is not used to deliver humidity to the samples during exposure; rather, humidity is provided by floating the sample substrate in a water bath. Further, as in the case of U.S. Pat. No. 4,817,477, the ""954 patent uses artifical light sources for exposure of the samples.
U.S. Pat. No. 3,686,940 discloses a water-cooled cylindrical mirror, which rejects infrared radiation in an ultraviolet test apparatus. In the ""940 patent, natural sunlight is not used.
A solar weathering device with control of sample temperature by cooling air is disclosed in U.S. Pat. No. 4,807,247. While this patent uses natural sunlight, a sample irradiance at accelerated levels of only up to 8 suns across the complete solar spectrum is employed.
U.S. Pat. No. 5,138,892 discloses accelerated light fastness testing of materials with xenon lamps and sample temperature control using airflow. Sample irradiance at accelerated UV levels of up to 8 suns (180 W/m2 between 300-400 nm) are attainable. This patent does not utilize natural sunlight in its testing of materials.
A weather test machine using xenon lamps and sample temperature and humidity control using airflow is disclosed in U.S. Pat. No. 5,646,358. Uniform sample irradiance at accelerated levels up to 1-3 suns (within the UV bandwidth) is attainable. This patent does not utilize natural sunlight in its weather test machine.
U.S. Pat. No. 5,153,780 discloses a dish reflector and method for concentrating moderate solar flux uniformly on a target plane, said dish having stepped reflective surface characterized by a plurality of ring-like segments arranged about a common axis, each segment having a concave spherical configuration.
3. The Need for Capabilities Beyond the Prior Art
There is a need for devising facilities for ultra-accelerated natural sunlight exposure testing of materials and devices under controlled weathering conditions that include several concurrent levels of temperature and/or relative humidity at very high levels of natural sunlight. This need is associated with the desirability to be able to predict the in-service lifetimes of said materials and devices from correlation""s derived between such realistically accelerated test results and those obtained during normal use conditions. Further, there is a need to conduct these ultra-accelerated exposure tests at irradiance exposure factors of from about 25 to 100 suns, wherein the irradiance is highly uniform. The need to conduct these ultra-accelerated natural sunlight exposure tests of materials and devices should exclude artificial light sources which invariably introduce uncertainties regarding realistic spectral content of the irradiance stress on samples being exposed. For example, the use of artificial light leads to unrealistic degradation mechanisms and failure modes of exposed materials caused by low wavelength ( less than 300 nm) photons that are not present in terrestrial solar spectra.
In light of the drawbacks of the foregoing prior art, a general object of the present invention is to provide the unique capability to carry out ultra-accelerated exposure testing of materials and devices under controlled conditions that include several concurrent levels of temperature and/or relative humidity at very high levels of natural sunlight, thereby permitting about a year""s worth of representative weathering, in terms of natural sunlight exposure, to be accumulated in from about 3 to about 10 days.
A further object of the present invention is to provide ultra accelerated exposure testing of materials and devices by controlling sample temperatures and humidities and demonstrating that reciprocity relationships are obeyed (i.e., level of applied accelerated stress does not change failure/degradation mechanism).
A yet further object of the present invention is to provide ultra-accelerated exposure testing of materials and devices that allows materials to be subjected to accelerated irradiance exposure factors of 25-100xc3x97 to provide about a year""s worth of representative weathering, in terms of natural sunlight exposure, to be accumulated in from about 3 to about 10 days.
A still further object of the invention is to provide a method of carrying out ultra-accelerated exposure testing of materials and devices utilizing a sample chamber that allows control of temperature and humidity during light exposure; wherein concentrated sunlight enters the chamber through an appropriate window, which may include quartz.
A further object yet still of the invention is to provide a method for carrying out ultra-accelerated exposure testing of materials and devices utilizing a cold mirror as a filter that reflects the ultraviolet/visible (UV/VIS) and transmits the near infrared (NIR) part of the solar spectrum, since the short wavelength (UV) light has been shown to be the predominant deleterious stress experienced by materials and devices during outdoor weathering.
Another object of the present invention is to provide a method of carrying out ultra-accelerated exposure testing of materials and devices under controlled weathering conditions, wherein conductive cooling of sample materials is provided by a water cooled substrate on to which samples are placed, and convective cooling is provided by blowing moist or dry air over the top surface of the samples, to provide high or low humidity to the samples during exposure of redirected concentrated sunlight into the exposure chamber to reduce the thermal load on the samples.
The invention is accomplished by the steps of: utilizing a solar concentrator to obtain elevated levels (25-100xc3x97) of concentrated sunlight with a uniform flux profile on the materials or samples being tested; splitting the solar spectrum into deleterious ultraviolet/visible (UV/VIS) light that enters the sample chamber; preventing concentrated near-infrared (NIR) radiation from entering the sample chamber to minimize undesirable thermal loading of material samples; and further control of temperature and/or relative humidity experienced by materials samples within the exposure chamber. The solar spectrum is split at a cut-off wavelength xcexcutoff such that UV/VIS consists of wavelengths less than xcexcutoff and VIS/NIR consists of wavelengths greater than xcexcutoff. Various combinations of concentrator designs (reflective and refractive), secondary reflectors, secondary concentrators, and turning mirrors can be used to provide the uniform flux. Additionally, the spectral splitting can be achieved at various points in the system through the use of coatings applied to any number of optical elements.
In terms of the best additional means for facilitating the general effect of ultra-accelerated natural sunlight exposure testing of materials, the facilities are as follows:
1) Multi-faceted concentrator design with facets having the following characteristics:
Facet centers located on a plane, parabola, sphere or other non-analytic shape;
Facet curvature that is flat, spherical, parabolic or aspheric; and
Facet reflector coatings designed to reflect UV light and transmit visible and IR, in the following configurations:
a) Multi-faceted concentrator with geometry and design of facets to produce uniform flux on a sample chamber located at or near the aim point of the facets
b) Multi-faceted concentrator with secondary reflector designed to deliver uniform flux to the sample chamber located near the center of the facet array
c) Multi-faceted concentrator with secondary concentrator designed to deliver uniform flux to the sample chamber located near the exit of the secondary
d) Multi-faceted concentrator with secondary reflector designed to deliver uniform flux to the sample chamber located below the secondary to allow a horizontal orientation of the sample chamber.
e) Multi-faceted concentrator with secondary reflector designed to deliver uniform flux to the sample chamber located below a turning mirror-placed near the center of the facet array.
f) Multi-faceted concentrator with secondary reflector designed to deliver uniform flux to the sample chamber below a turning mirror placed near the center of the facet array.
2) The multi-step concentrator of U.S. Pat. No. 5,153,780 Method and Apparatus for Uniformly Concentrating Solar Flux for PV Applications using a reflector coating designed to reflect UV light and transmit VIS and NIR in the following configurations:
a) Multi-step concentrator with secondary reflector designed to deliver uniform flux to the sample chamber located near the center of the multi-step concentrator
b) Multi-step concentrator with secondary concentrator designed to deliver uniform flux to the sample chamber located near the exit of the secondary
c) Multi-step concentrator with secondary reflector designed to deliver uniform flux to the sample chamber located below the secondary to allow a horizontal orientation of the sample chamber,
d) Multi-step concentrator with secondary reflector designed to deliver uniform flux to the sample chamber located below a turning mirror placed near the center of the multi-step concentrator
e) Multi-step concentrator with secondary reflector designed to deliver uniform flux to the sample chamber below a turning mirror placed near the center of the multi-step concentrator
3) Fresnel lens concentrator/heat mirror configurations that only permit the desired spectral range to be transmitted:
a) with heat mirror positioned above the top surface of the lens
b) with one or both surfaces of the lens having a heat mirror coating
c) with heat mirror positioned between the lens and the sample
d) with heat mirror positioned between the lens and sample, but oriented as a relay mirror to reflect the desired wavelengths to a position perpendicular to the plane of the lens
e) a two-stage Fresnel lens that interact as paired prisms to provide spectral selectivity
f) any of the above configurations combined with a secondary concentrator to achieve the desired flux uniformity
4) Holographic concentrator in the following configurations:
a) achieves both spectral splitting and uniform concentration in its fundamental design
b) provides spectral splitting in its fundamental design and uses a secondary concentrator to achieve the uniform flux
c) concentrates in its fundamental design and uses a secondary concentrator to achieve the uniform flux, but with a cold mirror coating on the secondary
d) provides uniform flux in its fundamental design and uses a cold mirror to achieve the spectral splitting
e) concentrates in its fundamental design and uses a secondary concentrator to achieve the uniform flux, but with a cold mirror placed between the lens and secondary to achieve the spectral splitting
5) Use of a 2D or 3D micro lens array to achieve flux uniformity and/or spectral splitting
6) An array of Fresnel lens facets can be used to achieve flux uniformity and in conjunction with a heat mirror or a cold mirror can provide spectral splitting